The present invention relates to a method and a sensor for determining an axle geometry, such as the track and camber on an axle of a motor vehicle. In general, an axle geometry is determined for each wheel fitted to an axle by means of determining a normal vector on the face of the wheel in relation to a common reference system.
An axle geometry on fitted wheels is usually measured during the final assembly of the motor vehicle or when working on the running gear of the vehicle in a workshop. In order to adjust the relative alignment of the wheels in relation to a reference system (e.g., the axle or the entire vehicle) by adhering to specific set point values, the actual, measured values must first be determined. With these actual values, the wheels are then precisely aligned via adjustment elements, in which case a new adjustment for checking can be calibrated once again.
Conventional methods measure axle geometry by using laser probes. For the illumination and signal recording, so-called light section methods are used. For each wheel to be measured, a plurality of laser probes is used, wherein each laser probe projects a line of light, in a radial direction, onto the outer tire cover area. Accordingly, a height profile of this linear range of the tire cover is recorded or calculated, the profile being in a common reference coordinate system.
By combining the test results of several sensors, the position of the normal vectors of a wheel can be determined. This determination can be made for individual wheels or simultaneously for all the wheels. The practical use of this method is to determine the track and camber values and to evaluate these values with respect to their deviation from required values.
There are various conventional methods used to determine the position of a tire relative to a reference system. For example, German Patent Publication DE-A-29 48 573 discloses a method and a device for the non-contact measurement of vehicle axles. Specifically, DE-A-29 48 573 describes contact-free measurements for determining an axle geometry on motor vehicles that, in addition to an illumination, include TV receiver tubes as receiving units. The method disclosed in DE-A-29 48 573 involves looking at the elliptical image of the outer rim diameter of a wheel, whereby the characteristics of the ellipse are determined. By changing the relative position between the television camera and the wheel, several images are recorded for evaluation purposes.
Further, a non-contact recording sensor is generally known from U.S. Pat. No. 4,745,469 which, using a structured light, illuminates the surface of a rotating tire with at least two contour lines: The contour lines are recorded, converted optoelectrically, and then evaluated by means of a video camera using the light that is reflected back from the tire. Distances are then measured by triangulation.
However, conventional measurement methods for analyzing an axle geometry of a wheel on a motor vehicle have a disadvantage in that they project only lines, circles, or ellipses, as geometrically structured light patterns, on a wheel to be measured. As such, the surface of the wheel cannot be recorded completely, even if a plurality of images are successively evaluated on the rotating wheel.
In particular, the influence of unevenness on a tire cover surface of a motor vehicle tire plays an important role in that it interferes with the evaluation of the image, or it does not allow a stable recording of the measured values. Unevenness on the surface of a tire includes, for example, the name of the make of the tire, dimensional specifications, rotating direction arrows, or other specifications. In order to determine a reference plane that is used to represent e.g., the outer face of a tire, interferences caused by unevenness that is not distributed evenly must be calculated out or averaged out around the tire. This is not possible with conventional methods in which either the tire is aligned by determining the position of the tire rim, or the tire cover is scanned or illuminated with only a few contour lines.